Method for determining retention of wire section in paper machine

ABSTRACT

A method and apparatus for determining the retention of a wire section in a paper machine. The mass flow of a paper web on the wire section is determined by means of web properties measured after the wire section. In addition, the mass flow of a headbox and/or white water is determined. The retention of the wire section is determined on the basis of the mass flows of the paper web and the headbox and/or the mass flow of the white water.

FIELD OF THE INVENTION

The invention relates to a method for determining the retention of awire section in a paper machine.

The invention further relates to an apparatus for determining theretention of a wire section in a paper machine.

The invention further relates to a method for adjusting the retention ofa wire section in a paper machine, in which method the retention of thewire section is adjusted by controlling the flow rate of a retentionagent.

The invention further relates to an apparatus for adjusting theretention of a wire section in a paper machine, the apparatus comprisingmeans for adjusting the retention of the wire section by controlling theflow rate of a retention agent.

BACKGROUND OF THE INVENTION

In a paper or board machine, the retention of the wire section expresseswhich proportion of the solids released from the headbox to the wiresection remains on the wire, and thus on the formed paper web, after thefiltration process. Since fibres are usually about 2 to 4 mm long, theirretention is intrinsically high, because they are considerably longerthan the 0.2 mm diameter of the openings in the wire of the wire sectionand thus they remain easily on the wire after mechanical filtration. Theretention of fibres is usually good, almost 100%. Since the diameter offines and filler components is typically 0.1-10 μm, they cannot attachto the wire after mechanical filtration until a fibre network is formedon the wire. The retention of fines and fillers on the wire is thereforeintrinsically very low. Fines and fillers are therefore bonded to thepaper web by means of different chemical retention agents, which allowretention of fillers to be increased to a level of 20-40%.

A current method that is widely used for determining the retention ofthe wire section in a paper or board machine is to calculate a totalretention R and a filler, or ash, retention R_(A) of the wire sectionusing the following formulae: $\begin{matrix}{R = {{\left( {1 - \frac{C_{ww}}{C_{hbx}}} \right) \cdot 100}\quad\%\quad{and}}} & (1) \\{{R_{A} = {{\left( {1 - \frac{C_{Aww}}{C_{Ahbx}}} \right) \cdot 100}\quad\%}},} & (2)\end{matrix}$where C_(ww) is the total consistency [g/l] of white water, C_(hbx) thetotal consistency [g/l] of a headbox, C_(Aww) the filler, or ash,consistency [g/l] of white water, and C_(Ahbx) the filler, or ash,consistency [g/l] of the headbox. The determining of retention accordingto formulae (1) and (2) is based on the idea that the amount of water tobe removed from the wire section to white water circulation isapproximately the same as the amount of water in the headbox. Since inreality about 95% of the water coming from the headbox to the wiresection is removed in the wire section, formulae (1) and (2) may bemodified to take this error into account. Assuming that K % of the watercoming from the headbox is removed in the wire section, formula (1), forexample, changes as follows: $\begin{matrix}{R = {{\left( {1 - {K\frac{C_{ww}}{C_{hbx}}}} \right) \cdot 100}\quad{\%.}}} & (3)\end{matrix}$

If water is removed from the wire section in two directions, i.e. a gapformer is used or the paper machine is provided with a dilution 20headbox, formula (1) and (2) take another form. For example, in a papermachine comprising a dilution headbox, the total retention of the wiresection is $\begin{matrix}{R = {{\left( {1 - \frac{C_{ww}}{{\left( {1 - d} \right) \cdot C_{hbx}} + {d \cdot C_{ww}}}} \right) \cdot 100}\quad\%}} & (4)\end{matrix}$and the filler retention of the wire section is $\begin{matrix}{{R_{A} = \left( {1 - \frac{C_{Aww}}{{\left( {1 - d} \right) \cdot C_{Ahbx}} + {d \cdot C_{Aww}}}} \right)},} & (5)\end{matrix}$where d is the dilution ratio, which expresses the ratio of the dilutionwater flow to the total flow at the headbox slice.

In this specification, the expression “the retention of the wiresection” is used, generally and without making a distinction, both forthe total retention of the wire section, which influences the entirestock to be fed to the wire section, and for the fibre retention, fillerretention and fines retention of the wire section, which relate to thesub-components, i.e. fibres, fillers and fibre-based fines, of thestock. Where a specific reference is made to the total retention of thewire section or to the retention of a specific sub-component of thestock, the exact name of the retention is used. The total retention ofthe wire section thus consists of the fibre, fines and fillerretentions.

The retention of the wire section in a paper machine is currentlycontrolled indirectly by controlling the total consistency of whitewater. The total consistency of white water is controlled by the dosageof a retention agent. This solution is based on the fact that whitewater contains most of the poorly retaining components of the shortcirculation, i.e. fines and fillers, and thus by controlling white waterconsistencies, a significant indicator of the state of short circulationcan be controlled at the same time. An increase in the amount of theretention agent increases retention, which is shown in a decrease in thewhite water consistency. A decrease in white water consistency reducesthe return of fines and fillers to the headbox and thus decreasesheadbox consistency as well. The solution is thus based on stabilizingthe white water consistency, or, when a dilution headbox is concerned,the dilution water consistency to a desired level.

The use of formulae (1) to (5) for determining retention is based on theidea that the measurements of the total consistency and the fillerconsistency of the headbox and the white water are error-free andrepresent the entire mass volume of the headbox and the white water. Inpractise it is extremely difficult to take a representative sample fromwhite water in particular, because the consistencies of the waterfractions leaving the wire section may vary significantly. White waterconsistency therefore almost always represents only a specific fractionof the waters in the wire section and thus it is biased in relation tothe mean value of the actual consistencies. The accuracy of white waterconsistency measurement can be improved by measuring the consistencyfrom a water fraction coming from a former roll, where the flow of thefraction, and thus its impact on retention, is the greatest. Anotheralternative for improving the accuracy of white water consistencymeasurement is to measure the consistency of white water at a point,such as a dilution water line of the dilution headbox, where thedifferent water fractions are already mixed. However, even by selectingthe location of the white water consistency measurement it is notpossible to avoid the dynamic phenomena relating to the measurement andmaking it difficult to determine a momentary retention of the wiresection with sufficient precision to allow retention control, forexample, to be carried out in an optimal manner, taking into account thehigh requirements set to the quality and efficiency of both the paper tobe manufactured and the manufacturing process.

EP 1,054,102 discloses a method for controlling the basis weight of apaper or board web in a paper or board machine comprising a dilutionheadbox in which method the basis weight of the web is measured and thewire retention of the machine is determined and, on the basis of these,the basis weight of the paper or board web is adjusted by changing theflow of the mass and the dilution water to the headbox. In the disclosedsolution wire retention is determined, in a manner known per se, on thebasis of the consistency of white water, mass flow, or the consistencyof the mass in the headbox.

DE 10,043,142, in turn, teaches a method for adjusting the dry mass ofpaper or board in a paper or board machine provided with a dilutionheadbox. In the method, a model representing the dry mass of the web isused for determining a calculated value for the dry mass of the web, thecalculated value being compared with a set value of the dry mass. Thedifference between the set value and the calculated value is used tocontrol the flow of the mass to be fed to the headbox and/or the flow ofthe dilution water or additive to be added to the mass to therebyregulate the dry mass of the web. According to an embodiment of thesolution disclosed in the publication, the additive to be added to themass consists of a retention agent, the flow of which is controlled inan attempt to adjust the retention degree of the wire section to aconstant level for controlling the dry mass of the web. The calculateddry mass of the web is determined on the basis of the mass consistency,dilution water consistency and flow, and the speed of the machine, themeasured basis weight of the web being used for calibrating the modelrepresenting the dry mass of the web.

FI applications 974,327 and 974,328 also describe methods for adjustingpaper properties by the dosage of the retention agent to be added to thestock. In the method disclosed in FI 974,327, a variable representingthe amount of fillers is measured from white water and, further, the ashcontent of the paper and/or of the headbox is measured, the amount offillers in the white water being adjusted by controlling the flow of theretention agent on the basis of the variable representing the amount offillers in the white water and the ash content of the paper beingadjusted by controlling the flow of the fillers on the basis of the ashcontent of the paper and/or of the headbox. According to this solution,a model is devised of the impact of the retention agent flow on theamount of the fillers in the white water and on the ash content of thepaper and of the impact of the filler flow on the ash content of thepaper and on the amount of fillers in the white water, this modellingbeing used for adjusting the amount of fillers in the white water andthe ash content of the paper by simultaneously controlling the retentionagent flow and the filler flow. In the method disclosed in FI 974,328,the basis weight of paper is measured and then adjusted by controllingthe flow of the machine stock on the basis of the measurement. Themethod comprises devising a model of the impact of the retention agentflow and the machine stock flow on the basis weight of the paper, themodelling being used for adjusting the basis weight of the paper bysimultaneously controlling the retention agent flow and the machinestock flow.

With all the above solutions there is, however, still the problem thatthe measured total or filler consistencies of white water or the headboxare assumed to represent the total mass volume of the headbox and thewhite water, which may prevent the precise determining of the retentionof the wire section and also the adjustment of the retention of the wiresection with sufficient precision in different production conditions.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the invention to provide a method and an apparatusfor determining the retention of a wire section with greater precisionthan before.

The method of the invention for determining the retention of the wiresection is characterized by determining the mass flow of a paper web onthe wire section by means of paper web properties measured after thewire section; determining the mass flow of a headbox and/or the massflow of white water; and determining the retention of the wire sectionon the basis of the mass flow of the paper web and the mass flow of theheadbox and/or the mass flow of the white water.

Further, the apparatus of the invention for determining the retention ofa wire section is characterized in that the apparatus comprises: meansfor determining the mass flow of a paper web on the wire section bymeans of paper web properties measured after the wire section; means fordetermining the mass flow of a headbox and/or the mass flow of whitewater; and means for determining the retention of the wire section onthe basis of the mass flow of the paper web and on the basis of the massflow of the headbox and/or the mass flow of the white water.

Still further, the method of the invention for adjusting the retentionof a wire section is characterized by determining the mass flow of apaper web on the wire section by means of paper web properties measuredafter the wire section; determining the mass flow of a headbox and/orthe mass flow of white water; determining the retention of the wiresection on the basis of the mass flow of the paper web and the mass flowof the headbox and/or on the basis of the mass flow of the white water;and adjusting the retention of the wire section by controlling the flowrate of the retention agent on the basis of the determined retention ofthe wire section.

Still further, the apparatus of the invention for adjusting theretention of the wire section is characterized in that the apparatusfurther comprises: means for determining the mass flow of a paper web onthe wire section by means of paper web properties measured after thewire section; means for determining the mass flow of a headbox and/orthe mass flow of white water; and means for determining the retention ofthe wire section on the basis of the mass flow of the paper web and themass flow of the headbox and/or the mass flow of the white water, andthat the retention of the wire section is arranged to be adjusted bycontrolling the retention agent flow on the basis of the determinedretention of the wire section.

According to a basic idea of the invention, the retention of the wiresection is determined by: determining the mass flow of the paper web onthe wire section by means of web properties measured after the wiresection; determining the mass flow of the headbox and/or the mass flowof the white water; and determining the retention of the wire section inthe paper machine on the basis of the mass flow of the paper web and themass flow of the headbox and/or the mass flow of the white water.According to an embodiment of the invention the wire section retentionto be determined represents the total retention of the wire section, themass flow of the paper web thus representing the total mass flow of thepaper web and, correspondingly, the mass flows of the headbox and thewhite water representing the total mass flows of the headbox and thewhite water. According to another embodiment of the invention, the wiresection retention to be determined is the filler retention of the wiresection, the mass flow of the paper web representing the filler massflow of the paper web and, correspondingly, the mass flows of theheadbox and the white water representing the filler mass flows of theheadbox and the white water. According to a third embodiment of theinvention, the total mass flow of the headbox is determined on the basisof the total mass flow of the paper web and the total mass flow of thewhite water; the total consistency of the headbox is determined on thebasis of the total mass flow of the headbox; and the total retention ofthe wire section is determined on the basis of the total consistency ofthe headbox. In a similar manner, also the total mass flow of the whitewater can be determined on the basis of the total mass flow of the paperweb and the total mass flow of the headbox; the total consistency of thewhite water is determined on the basis of the total mass flow of thewhite water and further the total retention of the wire section on thebasis of the total consistency of the white water. Further, the fillerconsistencies of the headbox and the white water and, on the basis ofthese, the corresponding filler retention can be determined in a similarmanner on the basis of the mass flows corresponding to the consistenciesin question.

The solution of the invention allows the retention of a wire section ofa paper machine to be determined with precision on the basis of the massflow of the headbox and/or the mass flow of the white water and the massflow of the paper web on the wire section, the mass flow of the paperweb being determined on the basis of paper web properties measured afterthe wire section. The solution of the invention is easy to implement ina corresponding manner for determining both the total retention and thefiller retention of the wire section. When necessary, the total massflow of the paper web and the total mass flow of the white water canalso be used for specifying the measurements of the total consistency ofthe headbox, which allows the total retention of the wire section to bedetermined with precision also on the basis of the consistencies of thewhite water and the headbox, if the consistency measurement is suspectedto be erroneous. In a similar manner, the measurement of the fillerconsistency of the headbox and the total consistency and fillerconsistency of the white water can be specified on the basis of the massflows corresponding to these consistencies. Further, because of theprecise value of the wire section retention, the retention of the wiresection can be adjusted with precision, taking into account both thequality properties of the paper to be manufactured and productionefficiency.

In this specification, the term “paper” refers not only to paper butalso to board and soft tissue and thus the disclosure relating to papermachines or paper manufacture is also valid for board or soft tissuemachines and for the manufacture of board and soft tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described with greater precisionwith reference to the accompanying drawings, in which

FIG. 1 is a schematic view of a paper machine and a short circulation ofthe paper machine;

FIG. 2 is a schematic block diagram illustrating the determining oftotal retention and, further, of adjusting total retention on the basisthe total retention obtained;

FIG. 3 is a schematic block diagram illustrating a second embodiment fordetermining total retention and, further, for adjusting total retentionon the basis of the determined total retention;

FIG. 4 is a schematic block diagram illustrating the adjusting of thetotal consistency of white water in a situation where a target value isdetermined for the total consistency of white water on the basis of atarget value determined for the total retention of the wire section; and

FIG. 5 is a schematic block diagram illustrating the adjusting of thefiller consistency of white water in a situation where a target value isdetermined for the filler consistency of the white water on the basis ofa target value determined for filler retention of the wire section.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic view of a paper machine 1 and a short circulationthereof. For the sake of clarity, FIG. 1 does not show the actual stockpreparation department of the paper machine 1, where the stock to besupplied to the paper machine 1 is prepared from one or more fibre rawmaterials and different fillers and additives. The paper machine 1comprises a headbox 2 from which stock is fed to a wire section 3 toform a paper web 4 onto a wire 3′. The paper web 4 is led to a presssection 5 and further to a drying section 6. From the drying section 6,the web is led to a reel 7. FIG. 1 also shows a measuring frame 8 placedupstream of the reel 7, the measuring frame 8 being provided with a backand forth moving measuring carriage having measurement means formeasuring quality properties of a finished paper web 4. The papermachine 1 may comprise other parts as well, such as a size press,calendar or a coating unit, which are not shown in FIG. 1, for the sakeof clarity. Further, since the operation of the paper machine 1 is knownto a person skilled in the art, it will not be discussed in greaterdetail here.

The stock coming from the stock preparation department of the papermachine I is fed to the headbox 2 through a wire pit silo 9. Stock fedto the wire pit silo 9, or in general to the short circulation of thepaper machine 1, is also called machine stock KM. In the wire pit silo9, water is added to the machine stock KM to adjust the consistency ofthe machine stock KM to the headbox consistency. The water to be addedto the machine stock KM is white water that has filtrated through thewire 3′ and although it contains some fibres, it mostly contains fillersand fines, whose retention to the paper web 4 is poor. If the headbox 2is a dilution headbox, the stock to be fed to the wire section 3 can bediluted by adding white water to the mass not only in the wire pit silo9 but also in the headbox 2. The collection of white water in the wirepit silo 9 and its mixing with the stock to be fed into the headbox 2forms the short circulation of the paper machine 1, the purpose of whichis to return the water leaving the wire section 3 and attach the solidsit contains back to the paper web 4, and to attenuate and even outinterference impulses arriving to the headbox. For the sake of clarity,FIG. 1 does not show the different cleaners, air discharge tanks andscreens, which also belong to the short circulation of paper machine andwhich are responsible for cleaning impurities and gases from the stockto be fed to the headbox 2. Since a person skilled in the art isfamiliar with the operation of a short circulation in a paper machine,it is not discussed in greater detail here.

The total retention R of the wire section 3 is determined on the basisof the mass balance of the wire section 3, i.e. the proportions of thetotal mass flow Q_(hbox) to be fed from the headbox 2 to the wiresection 3, the total mass flow Q_(pap) moving from the wire section 3 tothe press section 5, and the total mass flow Q_(ww) filtrating from thewire section 3 to the short circulation, which can be established whentwo out of the three total mass flows are known. On the basis of themass balance of the wire section 3, the total retention R can becalculated. Since the total mass flow Q_(pap) or the filler mass flowQ_(Apap) moving from the wire section 3 to the press section 5 cannot,in practise, be to determine on the wire section 3, the total retentionR and/or the filler retention R_(A) of the wire section 3 is determinedon the basis of the total mass flow Q_(papRL) [kg/s] or the filler massflow Q_(ApapRL) [kg/s] of the paper web 4 determined after the wiresection 3. The total mass flow Q_(pap) [kg/s] of the paper web 4 on thewire section 3 is thus determined on the basis of the total mass flowQ_(papRL) of the paper web 4 that is at the dry end of the paper machine1, and the filler mass flow Q_(Apap) [kg/s] of the paper web 4 on thewire section 3 is determined on the basis of the filler mass flowQ_(ApapRL) of the paper web 4 at the dry end of the paper machine 1. Thedry end of the paper machine 1 refers to the portion of the machine thatis downstream of the wire section 1, i.e. it includes the press section5 and the subsequent parts of the paper machine 1. The total mass flowand the filler mass flow of the paper web 4 at the dry end of the papermachine 1 can thus be determined also at another location that at themeasuring frame 8 placed just before the reel 7.

The total retention R of the wire section 3 can be determined withprecision when the total mass flow Q_(pap) of the paper web 4 on thewire section 3 and either the total mass flow Q_(hbox) of the headbox 2or the total mass flow Q_(ww) of the white water, or both, are known.The same principle can be applied in a corresponding manner to determinethe filler retention R_(A) of the wire section 3. The following exampleillustrates the determining of the total retention R of the wire section3 on the basis of the total mass flow Q_(ww) of the white water and thetotal mass flow Q_(pap) of the paper web 4.

The total mass flow Q_(ww) [kg/s] of the white water can be calculatedfrom the following formula: $\begin{matrix}{{Q_{ww} = \frac{f_{ww} \cdot C_{ww}}{1000}},} & (6)\end{matrix}$where f_(ww) represents the white water flow [l/s] and C_(ww) the totalconsistency of the white water. In practice the white water flow f_(ww)is approximately equal to the headbox 2 flow f_(hbx) [l/s], which can bedetermined for example directly by measuring the flow from the feedconduit 10 leading to the headbox 2 or indirectly on the basis of thepressure at the headbox 2 slice opening. The total mass flow Q_(papRL)[kg/s] of the paper web 4 at the dry end of the paper machine can becalculated using formula (7). It is assumed that only water is removedfrom the web as it proceeds in the paper machine from the press section5 to the reel 7, i.e. Q_(press)=Q_(papRL), where Q_(press) is the totalmass flow of the paper web 4 on the press section 5. $\begin{matrix}{{Q_{papRL} = {\frac{m_{papRL}}{1000} \cdot l_{papRL} \cdot v_{papRL}}},} & (7)\end{matrix}$where m_(papRL) represents the average dry weight [g/m²] of the paperweb 4, v_(papRL) the velocity [m/s] of the paper web 4, and l_(papRL)the width of the paper web 4 at the dry end of the paper machine 1. Thedry weight m_(papRL) of the paper web 4 is determined on the basis ofthe basis weight BW and moisture Moi of the paper web 4, which areusually measured with measuring devices provided in the measuringcarriage that traverses the paper web 4 back and forth in themeasurement frame 8. The velocity v_(papRL) of the paper web 4 can bedetermined on the basis of the speed of the closest operational group.The total width l_(pap) of the paper web 4 on the wire section 3 isl _(pap) =l _(press) +l _(trim)m,  (8)where l_(press) is the width of the paper web 4 running from the wiresection 3 to the press section 5 and l_(trim) is the width of the edgetrimmings removed from the paper web 4 in the wire section 3 during theproduction of the web, i.e.l _(press) =l _(hbx) −l _(trim),  (9)where l_(hbx) is the width of the headbox 2 slice opening. The widthl_(hbx) of the headbox 2 slice opening and the width l_(trim) of theedge trimmings can be determined in a manner known per se to a personskilled in the art. The total mass flow Q_(papRL) of the paper web 4 atthe dry end is scaled to correspond to the uncut or untrimmed total massflow Q_(hbx) of the headbox 2 by applying the following formula:Q _(pap) =Q _(papRL)·(l _(hbx) /l _(press))  (10)where Q_(pap) is the total mass flow of the paper web 4 on the wiresection 3. Assuming that the mass losses taking place in the presssection 5 and the drying section 6 are insignificant, the followingestimate can be generated for the headbox 2 mass flow Q_(hbx):{circumflex over (Q)} _(hbx) =Q _(ww) +Q _(pap),  (11)which can then be used for determining an estimate {circumflex over (R)}for the total retention R: $\begin{matrix}{\hat{R} = {\frac{Q_{pap}}{Q_{hbx}} = {{\frac{Q_{pap}}{Q_{ww} + Q_{pap}} \cdot 100}\quad{\%.}}}} & (12)\end{matrix}$

The disclosed solution allows the total retention R of the wire section3 to be determined with precision on the basis of two known total massflows, one of which is the total mass flow Q_(pap) of the paper web 4 onthe wire section 3, which is determined on the basis of the paper web 4properties measured after the wire section 3. The precise estimate{circumflex over (R)} of the total retention R allows the totalretention R of the wire section 3 to be adjusted under any productioncircumstances by controlling the retention agent flow f_(ra) to be addedto the stock to be fed to the headbox with precision in such a way thatthe quality and efficiency requirements set for the paper to bemanufactured and to the manufacturing efficiency can be attained.

FIG. 2 is a schematic block diagram illustrating, in accordance with theabove disclosure, the calculation of the estimate {circumflex over (R)}of the total retention R and the adjusting of the total retention Rbased on changing the dosage of the retention agent, when the retentionis being calculated directly from the mass balance of the wire section 3by applying the measurements taken at the dry end of the paper machine1. The total retention estimate {circumflex over (R)} and a set value ortarget value R_(SP) of the total retention are used for determining aset value f_(rasP) for the retention agent flow to control the retentionagent flow f_(ra). The retention agent flow f_(ra) can be changed forexample by changing the opening of the valve 11 shown in FIG. 1.

The estimate {circumflex over (R)} of the total retention R according toformula (12) is typically updated at intervals of 30 to 60 seconds. Theupdate interval depends on how fast the measuring carriage traveling inthe measuring frame 8 traverses the paper web 4, i.e. how often a newvalue is obtained for the dry weight m_(papRL) of the paper web 4. If afaster, i.e. more frequently determined, retention estimate is neededfor adjusting the total retention R, the estimate can be determined onthe basis of an estimate of the total mass flow Q_(hbx) of the headbox 2and an estimate of the total consistency C_(hbx) of the headbox 2, whichalso allow the accuracy of the consistency measurement of the headbox 2to be improved. Between the total mass flow Q_(hbx) of the headbox 2that corresponds to the precise total consistency C_(hbx) of the headbox2 and a mass flow {tilde over (Q)}_(hhx) calculated on the basis of aconsistency measurement deviating from the precise value there may be adifference equal to a term ΔQ_(hhx):ΔQ _(hbx) ={circumflex over (Q)} _(hbx) −Q _(hbx).  (13)

Assuming that the headbox 2 mass flow {circumflex over (Q)}_(hbx)estimated according to formula (11) comes very close to the actual flow,i.e. {circumflex over (Q)}_(hbx)≈Q_(hbx), the deviation ΔQ_(hbx) causedby the consistency measurement can be estimated as follows:Δ{circumflex over (Q)}_(hbx) ={tilde over (Q)} _(hbx) −{circumflex over(Q)} _(hbx),  (14)from which a consistency correction estimate ΔĈ_(hbx) can be calculated:$\begin{matrix}{{\Delta\quad{\hat{Q}}_{hbx}} = {\left. \frac{{f_{hbx} \cdot \Delta}\quad{\hat{C}}_{hbx}}{1000}\Leftrightarrow{\Delta\quad{\hat{C}}_{hbx}} \right. = {\frac{\Delta\quad{\hat{Q}}_{hbx}}{f_{hbx}} \cdot 1000.}}} & (15)\end{matrix}$

On the basis of the consistency correction estimate ΔĈ_(hbx) of formula(15), the consistency estimate of the headbox 2 isĈ _(hbx) ={tilde over (C)} _(hbx) −ΔĈ _(hbx),  (16)where {tilde over (C)}_(hbx) is the measured consistency of the headbox2 and ΔĈ_(hbx) is a correction term of the total consistency C_(hbx)estimated on the basis of the mass balance. Assuming that the termΔĈ_(hbx) changes slowly, depending on paper grade, for example, theaccuracy of the headbox 2 consistency to be used for calculating a fastestimate {circumflex over (R)} of the total retention R can be improved.The retention estimate {circumflex over (R)} can then be calculated inthe following manner, either according to formula (1) $\begin{matrix}{\hat{R} = {{\left( {1 - \frac{C_{ww}}{C_{hbx}}} \right) \cdot 100}\quad\%}} & (17)\end{matrix}$or by deducing from formula (1) $\begin{matrix}{\hat{R} = {{\frac{{f_{hbx} \cdot {\hat{C}}_{hbx}} - {f_{ww} \cdot C_{ww}}}{f_{hbx} \cdot {\hat{C}}_{hbx}} \cdot 100}\quad{\%.}}} & (18)\end{matrix}$

FIG. 3 is a block diagram schematically illustrating the adjusting ofthe total retention R when the retention is calculated using the totalconsistency of the headbox 2 determined as disclosed above.

The total retention R of the wire section 3 can be determined and theaccuracy of the measurement of the total consistency C_(ww) of the whitewater can be improved in a corresponding manner on the basis of thetotal mass flow Q_(pap) of the paper web 4 and the total mass flowQ_(hbx) of the headbox 2. Further, the total retention R of the wiresection 3 can be determined and the accuracy of the measurement of boththe total consistency C_(hbx) of the headbox 2 and the total consistencyC_(ww) of the white water can be improved in a corresponding manner onthe basis of the total mass flow Q_(pap) of the paper web 4, the totalmass flow Q_(hbx) of the headbox 2, and the total mass flow Q_(ww) ofthe white water.

The disclosed solution is fully applicable when, instead of the totalretention R of the wire section 3, the filler retention R_(A) of thewire section 3 is to be determined, in which case the wire section 3mass flows to be considered are the headbox 2 filler mass flow Q_(Ahbx),white water filler mass flow Q_(Aww), and paper web 4 filler mass flowQ_(Apap). The corresponding consistencies are: the headbox 2 fillerconsistency C_(Ahbx) and the white water filler consistency C_(Aww), andthe amount of fillers, i.e. ash, determined on the basis of the dryweight m_(papRL) of the paper and its filler, or ash, content A_(papRL).

According to an embodiment, the disclosed solution for determining thetotal retention R or the filler retention R_(A) of the wire section isused for calculating a target value C_(hbxSP), C_(wwSP), C_(AhbxSP) orC_(AwwSP) for the total headbox 2 or white water consistencies C_(hbx),C_(ww) or for the headbox or white water filler consistencies C_(Ahbx),C_(Aww). Since white water consistency is a typical adjustable variablecurrently used in retention adjustment, the following exampleillustrates the calculation of the target values for the total andfiller consistencies of white water. The target values for the totalconsistency and filler consistency of the headbox 2 can be calculated ina corresponding manner.

The paper to be manufactured is given a dry weight target valuem_(papSPRL)[g/m²] and filler content target value A_(papSPRL) [%].Assuming that the width of the paper web 4 at the measurement point isl_(papRL) [m] and its velocity is v_(papRL) [m/s], the target[kg/s] forthe total production at the dry end is $\begin{matrix}{{Q_{papSPRL} = {\frac{m_{papSPRL}}{1000} \cdot l_{papRL} \cdot v_{papRL}}},} & (19)\end{matrix}$which is scaled to correspond to a production target set to an untrimmedtotal mass flow Q_(pop) of the wire section 3:Q _(papSP) =Q _(papSPRL)·(l _(hbx) /l _(press))  (20)

Correspondingly, the production target [kg/s] relating to the amount offillers or ash in the paper isQ _(ApapSP) =A _(papSPRL) ·Q _(papSPR).  (21)

Similarly as in formula (12), when a target value R_(SP) is to beattained for the total retention R, the following formula must be valid:$\begin{matrix}{R_{SP} = {{\frac{Q_{papSP}}{Q_{papSP} + Q_{ww}} \cdot 100}{\%.}}} & (22)\end{matrix}$

From formula (22), the following can be solved: $\begin{matrix}{Q_{ww} = {\frac{{100 \cdot Q_{papSP}} - {R_{SP} \cdot Q_{papSP}}}{R_{SP}} = {\frac{Q_{papSP}}{R_{SP}/100} - {Q_{papSP}.}}}} & (23)\end{matrix}$

Since it is still true that $\begin{matrix}{{Q_{ww} = {\frac{f_{ww} \cdot C_{wwSP}}{1000} = {\frac{Q_{papSP}}{R_{SP}/100} - Q_{papSP}}}},} & (24)\end{matrix}$where C_(wwSP) is the target value for the total consistency [g/l] ofwhite water, the following target value is obtained for the white watertotal consistency: $\begin{matrix}{C_{wwSP} = {\frac{\left( {\frac{Q_{papSP}}{R_{SP}/100} - Q_{papSP}} \right) \cdot 1000}{f_{ww}}.}} & (25)\end{matrix}$

If, instead of the target retention R_(SP) of the total retention R, atarget value R_(ASP) for the filler retention R_(A) is to be set, anequation can be deduced in the above manner for calculating a targetvalue C_(AwwSP) [g/l] for the white water filler consistency C_(Aww):$\begin{matrix}{{C_{AwwSP} = \frac{\left( {\frac{Q_{ApapSP}}{R_{ASP}/100} - Q_{ApapSP}} \right) \cdot 1000}{f_{ww}}},} & (26)\end{matrix}$

-   -   where R_(ASP) is the target value [%] for the filler retention        RA. FIG. 4 is a block diagram schematically illustrating the        adjusting of the white water total consistency C_(ww) and FIG. 5        the adjusting of the white water filler consistency C_(Aww), the        target values C_(wwSP) and C_(AwwSP) for the consistencies being        calculated using the retention targets R_(SP) and R_(ASP).

In FIG. 1 and in the above examples, the quality properties of the paperweb 4 are measured with measuring devices or means arranged at themeasurement frame 8 located immediately before the reel 7. It is ofcourse evident that the necessary quality properties of the paper web 4may be measured at any point of the paper machine 1 after the wiresection 3. They may therefore be measured immediately after the wiresection 3, on the press section 5, on the drying section 6 or justbefore the reel 7, as shown in FIG. 1.

The functionalities shown in the block diagrams of FIGS. 2 to 5 arecarried out using equipment belonging to the automation system of thepaper machine 1, such as computers or other separately operatedcalculation and/or control and adjustment equipment. Functionalitiesrequiring calculation are preferably implemented by means of software.Each functionality of FIG. 2 to 5 involving calculation can naturally beexecuted in separate calculation units or by separate devices, butpreferably all the necessary calculation functionalities are carried outin a single, central computing unit or device.

The drawings and the related specification are only meant to illustratethe idea of the invention. The details of the invention may vary withinthe scope of the claims. The method disclosed above and its embodimentscan therefore be applied in a corresponding manner to a fibre-basedfines material, although the examples only discuss the determining ofthe total retention and the filler retention and the use of theseretentions for adjusting retention. In addition to actual retentionadjustment, an accurate retention estimate enables to improve theoperation of other applications requiring retention data.

1. A method for determining the retention of a wire section in a papermachine, comprising: determining the mass flow of a paper web on thewire section by means of paper web properties measured after the wiresection; determining the mass flow of a headbox and/or the mass flow ofwhite water; and determining the retention of the wire section on thebasis of the mass flow of the paper web and the mass flow of the headboxand/or the mass flow of the white water.
 2. A method according to claim1, wherein the retention of the wire section to be determined in thepaper machine represents a total retention of the wire section.
 3. Amethod according to claim 2, wherein the mass flow of the paper web tobe determined represents a total mass flow of the paper web on the wiresection, the mass flow being determined on the basis of productioncorresponding to the dry weight of the paper web.
 4. A method accordingto claim 3, comprising: determining the mass flow of the white water,the mass flow of the white water to be determined representing a totalmass flow of the white water determined on the basis of a headbox flowrate or a white water flow rate and a total consistency of the whitewater.
 5. A method according to claim 4, further comprising: determininga total mass flow of the headbox on the basis of the total mass flow ofthe paper web and the total mass flow of the white water; determining atotal consistency of the headbox on the basis of the total mass flow ofthe headbox; and by determining a total retention of the wire section onthe basis of the total consistency of the headbox.
 6. A method accordingto claim 3, comprising: determining the mass flow of the headbox, themass flow of the headbox to be determined representing the total massflow of the headbox determined on the basis of the flow rate of theheadbox and the total consistency of the headbox.
 7. A method accordingto claim 6, further comprising: determining the total mass flow of thewhite water on the basis of the total mass flow of the paper web and thetotal mass flow of the headbox; determining the total consistency of thewhite water on the basis of the total mass flow of the white water; andby determining the total retention of the wire section on the basis ofthe total consistency of the white water.
 8. A method according to claim3, comprising: determining both the mass flow of the headbox and themass flow of the white water, the headbox mass flow to be determinedrepresenting the total mass flow of the headbox and the white water massflow to be determined representing the total mass flow of the whitewater.
 9. A method according to claim 8, comprising: determining thetotal mass flow of the white water on the basis of the headbox flow rateor the white water flow rate and the measured total consistency of thewhite water and by determining the total mass flow of the headbox on thebasis of the headbox flow rate and the measured total consistency of theheadbox.
 10. A method according to claim 8, further comprising:determining the total mass flow of the headbox on the basis of the totalmass flow of the paper web and the total mass flow of the white water;determining the total consistency of the headbox on the basis of thetotal mass flow of the headbox; determining the total mass flow of thewhite water on the basis of the total mass flow of the paper web and thetotal mass flow of the headbox; determining the total consistency of thewhite water on the basis of the total mass flow of the white water; andby determining the total retention of the wire section on the basis ofthe total consistency of the headbox and the total consistency of thewhite water.
 11. A method according to claim 2, further comprising:determining a target value for the total consistency of the headbox onthe basis of a total production target set for the production after thewire section or the total production after the wire section and a targetvalue for the total retention of the wire section.
 12. A methodaccording to claim 2, further comprising: determining a target value forthe total consistency of the white water on the basis of a totalproduction target set for the production after the wire section or thetotal production after the wire section and a target value for the totalretention of the wire section.
 13. A method according to claim 1,wherein the retention of the wire section to be determined in the papermachine represents a filler retention of the wire section.
 14. A methodaccording to claim 13, wherein the mass flow of the paper web to bedetermined represents the filler mass flow of the paper web on the wiresection and it is determined on the basis of the amount of filler in thepaper web.
 15. A method according to claim 14, comprising: determiningthe mass flow of the white water, the mass flow of the white water to bedetermined representing a filler mass flow of the white water determinedon the basis of the headbox flow rate or the white water flow rate andthe filler consistency of the white water.
 16. A method according toclaim 15, further comprising: determining a filler mass flow of theheadbox on the basis of the filler mass flow of the paper web and thefiller mass flow of the white water; determining a filler consistency ofthe headbox on the basis of the filler mass flow of the headbox; and bydetermining a filler retention of the wire section on the basis of thefiller consistency of the headbox.
 17. A method according to claim 14,comprising: determining the mass flow of the headbox, the headbox massflow to be determined representing the filler mass flow of the headboxdetermined on the basis of the headbox flow rate and the fillerconsistency of the headbox.
 18. A method according to claim 17, furthercomprising: determining the filler mass flow of the white water on thebasis of the filler mass flow of the paper web and the filler mass flowof the headbox; determining the filler consistency of the white water onthe basis of the filler mass flow of the white water; and by determiningthe filler retention of the wire section on the basis of the fillerconsistency of the white water.
 19. A method according to claim 14,comprising: determining both the mass flow of the headbox and the massflow of the white water, the headbox mass flow to be determinedrepresenting the filler mass flow of the headbox and the white watermass flow to be determined representing the filler mass flow of thewhite water.
 20. A method according to claim 19, comprising: determiningthe filler mass flow of the white water on the basis of the headbox flowrate or on the basis of the white water flow rate and the measured whitewater filler consistency and by determining the headbox filler mass flowon the basis of the headbox flow rate and the measured fillerconsistency of the headbox.
 21. A method according to claim 19, furthercomprising: determining the filler mass flow of the headbox on the basisof the filler mass flow of the paper web and the filler mass flow of thewhite water; determining the filler consistency of the headbox on thebasis of the filler mass flow of the headbox; determining the fillermass flow of the white water on the basis of the filler mass flow of thepaper web and the filler mass flow of the headbox; determining thefiller consistency of the white water on the basis of the filler massflow of the white water; and by determining the filler retention of thewire section on the basis of the filler consistency of the headbox andthe filler consistency of the white water.
 22. A method according toclaim 13, further comprising: determining a target value for the fillerconsistency of the headbox on the basis of a production target set forthe filler production after the wire section or on the basis of thefiller production after the wire section and the target value for thefiller retention of the wire section.
 23. A method according to claim13, further comprising: determining a target value for the fillerconsistency of the white water on the basis of the production target setfor the filler production after the wire section or on the basis of thefiller production after the wire section and the target value for thefiller retention of the wire section.
 24. A method for adjusting theretention of a wire section in a paper machine, in which method theretention of the wire section is adjusted by controlling the flow rateof a retention agent, comprising: determining the mass flow of a paperweb on the wire section by means of paper web properties measured afterthe wire section; determining the mass flow of a headbox and/or the massflow of white water; determining the retention of the wire section onthe basis of the mass flow of the paper web and the mass flow of theheadbox and/or on the basis of the mass flow of the white water; andadjusting the retention of the wire section by controlling the flow rateof the retention agent on the basis of the determined retention of thewire section.
 25. An apparatus for determining the retention of a wiresection in a paper machine, the apparatus comprising: means fordetermining the mass flow of a paper web on the wire section by means ofthe paper web properties measured after the wire section; means fordetermining the mass flow of a headbox and/or the mass flow of whitewater; and means for determining the retention of the wire section onthe basis of the mass flow of the paper web and on the basis of the massflow of the headbox and/or the mass flow of the white water.
 26. Anapparatus according to claim 25, wherein the apparatus is configured todetermine the total retention of the wire section.
 27. An apparatusaccording to claim 26, wherein the mass flow of the paper web to bedetermined represents the total mass flow of the paper web on the wiresection, the headbox mass flow to be determined represents the totalmass flow of the headbox and/or the white water mass flow to bedetermined represents the total mass flow of the white water.
 28. Anapparatus according to claim 27, wherein the apparatus is configured todetermine the total mass flow of the paper web on the basis of theproduction corresponding to the dry weight of the paper web.
 29. Anapparatus according to claim 27, wherein the apparatus is configured todetermine the total mass flow of the headbox on the basis of the flowrate of the headbox and the total consistency of the headbox and/or theapparatus is configured to determine the total mass flow of the whitewater on the basis of the headbox flow rate or the white water flow rateand the total consistency of the white water.
 30. An apparatus accordingto claim 29, wherein the apparatus further comprises means fordetermining the total mass flow of the headbox on the basis of the totalmass flow of the paper web and the total mass flow of the white water;means for determining the total consistency of the headbox on the basisof the total mass flow of the headbox; and means for determining thetotal retention of the wire section on the basis of the totalconsistency of the headbox.
 31. An apparatus according to claim 29,wherein the apparatus further comprises means for determining the totalmass flow of the white water on the basis of the total mass flow of thepaper web and the total mass flow of the headbox; means for determiningthe total consistency of the white water on the basis of the total massflow of the white water; and means for determining the total retentionof the wire section on the basis of the total consistency of the whitewater.
 32. An apparatus according to claim 29, wherein the apparatusfurther comprises means for determining the total mass flow of theheadbox on the basis of the total mass flow of the paper web and thetotal mass flow of the white water; means for determining the totalconsistency of the headbox on the basis of the total mass flow of theheadbox; means for determining the total mass flow of the white water onthe basis of the total mass flow of the paper web and the total massflow of the headbox; means for determining the total consistency of thewhite water on the basis of the total mass flow of the white water; andmeans for determining the total retention of the wire section on thebasis of the total consistency of the headbox and the total consistencyof the white water.
 33. An apparatus according to claim 26, wherein theapparatus further comprises means for determining a target value for thetotal consistency of the headbox on the basis of a total productiontarget set for the production after the wire section or the totalproduction after the wire section and a target value for the totalretention of the wire section.
 34. An apparatus according to claim 26,wherein the apparatus further comprises means for determining a targetvalue for the total consistency of the white water on the basis of thetotal production target for the production after the wire section or thetotal production after the wire section and the target value for thetotal retention of the wire section.
 35. An apparatus according to claim25, wherein the apparatus is configured to determine the fillerretention of the wire section.
 36. An apparatus according to claim 35,wherein the mass flow of the paper web to be determined represents afiller mass flow of the paper web on the wire section, the headbox massflow to be determined represents a filler mass flow of the headboxand/or the mass flow of the white water to be determined represents afiller mass flow of the white water.
 37. An apparatus according to claim36, wherein the apparatus is configured to determine the filler massflow of the paper web on the basis of the amount of the filler in thepaper web.
 38. An apparatus according to claim 36, wherein the apparatusis configured to determine the filler mass flow of the headbox on thebasis of the headbox flow rate and the filler consistency of the headboxand/or that the apparatus is configured to determine the filler massflow of the white water on the basis of the headbox flow rate or thewhite water flow rate and the filler consistency of the white water. 39.An apparatus according to claim 38, wherein the apparatus furthercomprises means for determining the filler mass flow of the headbox onthe basis of the filler mass flow of the paper web and the filler massflow of the white water; means for determining the filler consistency ofthe headbox on the basis of the filler mass flow of the headbox; andmeans for determining the filler retention of the wire section on thebasis of the filler consistency of the headbox.
 40. An apparatusaccording to claim 38, the apparatus further comprising means fordetermining the filler mass flow of the white water on the basis of thefiller mass flow of the paper web and the filler mass flow of theheadbox; means for determining the filler consistency of the white wateron the basis of the filler mass flow of the white water; and means fordetermining the filler retention of the wire section on the basis of thefiller consistency of the white water.
 41. An apparatus according toclaim 38, the apparatus further comprising: means for determining thefiller mass flow of the headbox on the basis of the filler mass flow ofthe paper web and the filler mass flow of the white water; means fordetermining the filler consistency of the headbox on the basis of thefiller mass flow of the headbox; means for determining the filler massflow of the white water on the basis of the filler mass flow of thepaper web and the filler mass flow of the headbox; means for determiningthe filler consistency of the white water on the basis of the fillermass flow of the white water; and means for determining the fillerretention of the wire section on the basis of the filler consistency ofthe headbox and the filler consistency of the white water.
 42. Anapparatus according to claim 35, the apparatus further comprising meansfor determining a target value for the filler consistency of the headboxon the basis of a production target set for the filler production afterthe wire section or on the basis of the filler production after the wiresection and the target value for the filler retention of the wiresection.
 43. An apparatus according to claim 35, the apparatus furthercomprising means for determining a target value for the fillerconsistency of the white water on the basis of the production target setfor the filler production after the wire section or on the basis of thefiller production after the wire section and the target value for thefiller retention of the wire section.
 44. An apparatus for adjusting theretention of a wire section in a paper machine, the apparatuscomprising: means for determining the mass flow of a paper web on thewire section by means of paper web properties measured after the wiresection; means for determining the mass flow of a headbox and/or themass flow of white water; means for determining the retention of thewire section on the basis of the mass flow of the paper web and the massflow of the headbox and/or the mass flow of the white water, and meansfor adjusting the retention of the wire section by controlling theretention agent flow on the basis of the determined retention of thewire section.